As raw materials for cosmetics, paint, coating materials and polishing slurry for semiconductor wafers, raw materials of high purity with a minimal concentration of impurities, such as inorganic particles synthesized by a vapor phase method, such as fumigation, (hereinafter referred to as "vapor phased inorganic particles") have been used. However, as the vapor phased inorganic particles are intensive in secondary coagulation, when the vapor phased inorganic particles are dispersed in the water, coagula of such vapor phased inorganic particles should be destroyed and fused in the water. If the coagula are not completely destroyed or fused, problems, such as increase in viscosity of the aqueous dispersion slurry as the time passes, gelation and consequent loss of fluidity, and sedimentation and separation of the coagula. As a result, the aqueous dispersion slurry of inorganic particles can no longer be applied to those uses as described above.
As a method of dispersing the vapor phased inorganic particles in an aqueous medium, a method using dispersion equipment of high-speed mixing type (e.g., whirling blender, and high shearing mixer) as disclosed in U.S. Pat. No. 5,116,535 (Japanese Unexamined Patent Publication No. 3-50112) has been known. Also, a method using equipment combining a powder introduction type mixer/disperser (e.g., jet stream mixer) with a toothed colloid mill, a dissolver or a skim mixer (Nippon Aerosil Co., Ltd.; Catalog No. 19: How to Handle Aerosil, p. 38) has also been known. However, both of these methods are disadvantageous in that a long-time processing is required and that vapor phased inorganic particles can not be completely destroyed or fused.
As regards the method of dispersing pulverized silica or fumed silica in an aqueous medium, a method of pulverizing and dispersing the vapor phased inorganic particles using a high-pressure homogenizer to 100 nm or less in average secondary particle diameter has been disclosed in the Japanese Unexamined Patent Publication No. 9-142827 (opened on Jun. 3, 1997) and in the Japanese Unexamined Patent Publication No. 9-193004 (opened on Jul. 29, 1997), which were opened after the date of application of the Japanese Patent Application No. 9-132881 over which the present invention claims priority. However, these publications contain no description of the dispersion of inorganic particles other than the pulverized silica and the fumed silica.
Under these circumstances, development of a method of producing the aqueous dispersion slurry of various inorganic particles has been wanted.